In the anatomy of the human heart, the left atrium receives oxygenated blood from the lungs through the pulmonary vein. The mitral valve separates the left atrium from the left ventricle. During diastole, as the contraction triggered by the sinoatrial node progresses through the atria, oxygenated blood passes through the mitral valve into the left ventricle. In this phase, the aortic valve leading into the ascending aorta closes, allowing the left ventricle to fill with blood. A similar flow of venous blood occurs from the right atrium through the pulmonary valve to the right ventricle. Once the ventricles are full, they contract during the systolic phase and pump blood out of the heart. During systole, the mitral valve closes and the aortic valve opens, thus preventing blood from regurgitating into the left atrium and forcing blood into the aorta, and from there throughout the body. Because of the high pressures associated with the left ventricle during systole, proper functioning of the mitral valve to prevent blood from flowing back through the system is extremely important.
The various anatomical components of the left ventricle LV and mitral valve MV are depicted in FIG. 1 as seen in vertical cross-section along an anterior-posterior plane. The mitral annulus MA comprises a fibrous ring encircling the orifice between the left atrium LA and the left ventricle LV. The average human mitral annular cross-sectional area is 5-11 cm2. The anterior aspect of the mitral annulus MA forms a part of the “cardiac skeleton” and includes left and right fibrous trigones, LT and RT. FIG. 3 illustrates the mitral valve from the left atrium as exposed during surgery. The mitral valve is a bicuspid valve having a posterior leaflet PL that cooperates with an anterior leaflet AL. The left trigone LT and right trigone RT are indicated at the junction points of the anterior leaflet AL and posterior leaflet PL. These junction points are also known as commissures between the leaflets. The posterior aspect of the mitral annulus MA, in contrast to the anterior aspect, consists mainly of muscular tissue of the outer wall of the heart.
With reference again to FIG. 1, a pair of papillary muscles P1 and P2 attach to the lower portion of the interior wall of the left ventricle LV. Chordae tendineae CT extend between and link the papillary muscles P1 and P2 and free edges of the anterior and posterior leaflets AL and PL. The chordae tendineae are string-like in appearance and are sometimes referred to as “heart strings.” Although not shown in the drawing, chordae tendoneae CT extend between each of the papillary muscles P1 and P2 and both leaflets. Contraction of the papillary muscles P1 and P2 pulls the chordae tendoneae CT, which in turn pulls the leaflets open, and when the muscles relax the chordae tendonae become slack, allowing the leaflets to come together or “coapt.” As seen in FIG. 1, the leaflets coapt along a substantial surface area in the normal functioning heart, with the free edges of the leaflets mutually bending toward the left ventricle LV.
As seen in FIG. 1, and for purpose of discussion, the mitral annulus MA of a normal, healthy heart lies generally in a datum plane 20 defined perpendicular to the average blood flow direction 22 through the mitral valve MV. Although a typical mitral annulus MA may be three-dimensional, the datum plane 20 is representative of the relative positions of the anterior and posterior sides of the annulus.
In many developed countries, congestive heart failure is a leading cause of hospitalization and death, and its incidence is increasing. When imperfections in the mitral valve allows blood to flow backward into the left atrium, known as secondary mitral regurgitation, the left ventricle must pump progressively harder to circulate blood throughout the body, which in turn promotes congestive heart failure. Heart transplantation is considered a standard treatment for select patients with severe congestive heart failure and end-stage heart disease, but only a small number of donor hearts are available and there are severe surgical risks for weaker patients. Accordingly, alternative medical and surgical strategies are evolving to treat such conditions.
One typical cause of mitral regurgitation is malformation of the mitral annulus MA along the more flexible posterior aspect of the annulus. As seen in FIG. 2, some patients experience a depression h of the posterior aspect of the annulus caused by dilation of the left ventricle LV. Dilation of the left ventricle LV is a symptom associated with mitral regurgitation in patients with iopathic dilated cardiomyopathy or ischemic cardiomyopathy, and in patients with long-standing valvular regurgitation from other etiologies such as myxomatous disease, endocarditis, congenital defects, or rheumatic valvular disease. FIG. 3 illustrates the subsequent loss of coaptation between the posterior and anterior leaflets AL and PL from this posterior aspect depression, as seen from above.
As seen in FIG. 2, dilation of the left ventricle LV generally increases the distance between the papillary muscles P1 and P2 and the mitral annulus MA. This in turn increases the tension in the chordae tendonae CT. The droop or depression of the posterior aspect of the annulus below the datum plane 20 by the distance h in combination with the increased tension in the chordae reduces the ability of the leaflets to come together during systole.
Various interventions have been used to alter the size of the regurgitant orifice area. Annuloplasty rings have been developed in various shapes and configurations over the years to correct mitral regurgitation and other conditions which reduce the functioning of the valve. For example, Carpentier, et al. in U.S. Pat. No. 4,055,861 disclosed two semi-rigid supports for heart valves, one of which being closed (or D-shaped) and the other being open (or C-shaped). In the closed configuration, the ring is generally symmetric about an anterior-posterior plane, and has a convex posterior side and a generally straight anterior side. U.S. Pat. Nos. 5,104,407, 5,201,880, and 5,607,471 all disclose closed annuloplasty rings that are bowed slightly upward on their anterior side. Because the anterior aspect of the mitral annulus MA is fibrous and thus relatively inflexible (at least in comparison to the posterior aspect), the upward curve in the anterior side of each ring conforms that ring more closely to the anatomical contour of the mitral annulus, and thus reduces undue deformation of the annulus.
In general, conventional annuloplasty rings are intended to restore the original configuration of the mitral annulus MA, or in other words bring the annulus as close as possible back to the datum plane 20 as seen in FIG. 1. When correcting a condition as seen in FIG. 2, high stresses are created in the sutures connecting the annuloplasty ring to posterior aspect of the annulus because the ring “pulls” the annulus upward. The stresses sometimes result in the dehiscence or separation of the ring from the annulus at this location because the sutures pull through the tissue.
It should be noted here that correction of the aortic annulus requires a much different ring than with a mitral annulus. For example, U.S. Pat. Nos. 5,258,021 and 6,231,602 disclose sinusoidal or so-called “scalloped” annuloplasty rings that follow the up-and-down shape of the three cusp aortic annulus. Such rings would not be suitable for correcting a mitral valve deficiency.
While good results in the treatment of congestive heart failure and mitral regurgitation have been obtained in the preliminary applications of the above-described methods and apparatuses, it is believed that these results can be significantly improved. Specifically, it would be desirable to produce a mitral annuloplasty ring that can reduce stresses associated with the implantation of conventional rings.